What is "Android-App" traffic source?

Traffic source android-app corresponds to in-app navigation on the Android platform to your site. 

For instance, if an android user uses an application and clicks on a link pointing to your site, it will be categorized in the application-integrated browser (Android-app). 

For your information, you can retrieve the detailed origins of those referrer sites through Referrer Sites > Domains and select android app > URLs. 

Please note that you cannot find the same referrer for iOS, because their navigation process is not the same.